1. Field of the Invention
The present invention relates to a printed circuit board, and more particularly to a printed circuit board of a multi-layered structure in which each of the multi-layers has the same heat expansion coefficient and a middle layer of the multi-layers is exposed outside to form an electrical interconnection portion. In specific, the present invention relates to a printed circuit board in which a source PCB and a connector PCB have a multi-layered structure and the connector PCB that connects the source PCB with the control PCB is integrated with the source PCB.
2. Description of the Related Art
Generally, printed circuit boards (PCBs) on which electrical interconnections are printed and various devices are mounted are being used in most of electronic appliances.
Like other electronic apparatus, a liquid crystal display (LCD) also uses such PCBs for mounting the driving units displaying images on the LCD panel.
Generally, an LCD includes an LCD panel including a TFT substrate and a color filter substrate, a light source, and driving units which include a gate PCB, a source PCB, and a control PCB.
Gate PCB is used for driving gates of a plurality of thin film transistors formed on TFT substrate and source PCB is used for driving sources of the plurality of thin film transistors.
Generally, a control PCB is provided in addition to the gate PCP and the source PCB.
In LCDs that a gate PCB, a source PCB, and a control PCB are separate from each other, a connecting member such as flexible printed circuit (FPC) electrically connects the gate PCB, the source PCB and the control PCB.
When an FPC connects the source PCB and the control PCB, both terminals of the FPC are respectively connected to ports respectively mounted on the edges of the source PCB and gate PCB by a tape automated bonding (TAB) tape or by soldering. A TAB tape or soldering requires bonding, which lowers the productivity in general.
To resolve the above-described drawback, a source PCB integrated with a connector PCB is provided. The source PCB and the connector PCB respectively have a multi-layered structure.
A multi-layered structure PCB includes a flexible middle layer, a pair of rigid layers disposed on both surfaces of the flexible middle layer, and at least one conductive pattern layer disposed between the flexible middle layer and the rigid layer.
To connect the source PCB and the connector PCB, a flexible FPC is used and connected to a conductive pattern layer of each of the source PCB and the connector PCB. Here, both connectors of FPC extend not to the whole surface of the conductive pattern layer of the two PCBs but to edge portions of the PCBs facing each other, creating a step at the connection of FPC due to the thickness of the FPC. Such a step weakens the adhesive force between the conductive pattern layer and the overlying rigid layer.
Also, since the flexible middle layer of polyimide has a heat expansion coefficient different from the rigid layer of FR4 placed on both surfaces of the middle layer, the rigid layer and the flexible layer are delaminated during the soldering process of high temperature, lowering the reliability of the product.
In addition, polyimide that is more expensive than FR4 increases the total production cost of PCB.